Optical Tweezers

A Brief History: The physics behind the relatively young technique of optical tweezing has been known for centuries. In the seventeenth century, Johannes Kepler theorized that solar irradiance caused the tail of a comet to point away from the sun, and in 1873, James Clerk Maxwell proved theoretically that light can exert a force on matter (commonly know as radiation pressure, the "light force"). Sixty years later, Otto R. Frisch was able to deflet a beam of sodium atoms by bombarding the beam with light from a sodium lamp. In 1975, Hansch and Schalow proposed the idea of using lasers to trap atoms; a decade later, Steven Chu of Bell Laboratories was able to achieve three dimensional cooling in a technique nicknamed "optical molasses" (he would go on to win the Nobel Prize in Physics for the laser cooling and trapping of atoms with William Phillips and Claude Cohen-Tannoudji). These earlier techniques were all either optical two-beam traps or required an external force to be supplied by gravity or an electric field (these external forces provided trap stability). This paved the way for a single-beam gradient trap now known as optical tweezers . A year after Vladilen S. Letokov's proposal of atmoic trapping by light beams, Arthur Ashkin at Bell Laboratories accelerated transparent latex shpheres suspended in water using a laser beam (1986). By 1987, Arthur Ashkin of Bell Laboratories was able to trap living biological objects with a single laser, bringing the technique of optical tweezers, or optical trapping, to the scientific world.

Optical Trapping Theory: There are three general schemes for describing the physics behind optical tweezing. Generally, the particles trapped are dielectric (a substance with a conductivity less than a millionth of a siemens) spheres of polystyrene silica or silica and are polarized when subjected to an applied electric field. The ratio of sphere diameter to the wavelength of trapping length (d/lambda) is referred to as Z. In general, the less refractile or the smaller the particle, the less trapping force there is per watt.

Applications: The implications of a device capable of non-invasive translational manipulation of particles in the nanometer to micrometer range have not been fully realized by the scientific community; however, a flood gate of biological applications of the optical tweezer has greatly accelerated research in the field.

Construction:the principles that govern an ideal optical trap cannot accurately reflect the the experimental setup due to the physical limitations of the laser (its divergent nature and Gaussian profile). Consequently, there is an intensive process of alignment of numerous optical components. For the current setup, the materials for the trap include:

Other Materials:

Research: Much attention has been given to optimization of trapping capabilities. The Numerical Aperture Quandary (NAQ) is the 'real life' trade-off between numerical aperture, trapping efficiency and trapping depth. The difficulty in many of these procedures has been the formulation of an effective measurement of trapping force. Past examples of quantifying the trapping power include the treatment of a trap as a spring with a stiffness value k. A dielectric particle at some position x from the focus of the laser beam experiences an attractive force towards the focus, and this restoring foce is proportional to the distance between the center of the sphere and the focus of the laser. The force is commonly calibrated through the application of a known force, such as gravity or viscous drag (related to sphere radius r, velocity v, liquid viscosity n and viscous drag coefficient y, F vis = yv = 6ónrv). Additional indirect methods are employed, including the subjection of a trapped particle to a certain stress trap or vigorous osciallation at constant frequency and amplitude.

Abstract:A Study of Trapping Efficiency in Optical Tweezers as a Function of Beam Profile Yiyi Deng, Ward Melville High School, East Setauket, NY; Harold Metcalf and John No, Laser Teaching Center, Department of Physics and Astronomy, Stony Brook University.

Optical tweezers are an important application of James Clerk Maxwell's 1873 abstraction of radiation pressure, or the ability of light to exert a force. More than a century was to pass before Arthur Ashkin of Bell Laboratories became the first to exploit the theory of radiation pressure by demonstrating optical confinement of transparent latex spheres suspended in aqueous media. Optical tweezers are capable of non-invasive trapping and manipulation of a variety of dielectric nanoparticles and have many interesting applications, especially in biomedical fields. These include chromosome dissection, kinetic studies of DNA and other nucleic acids, DNA injection, controlled cell fusion, microsurgery and manipulation of cells in vivo, in vitro fertilization and finally force measurements of kinesin, organelles, and other sub cellular structures associated with cellular transport and adhesion.

Tweezing theory depends on the ratio of the diameter of the sphere being trapped to the wavelength of the laser light. The ratio of the yeast cell diameter to the wavelength is approximately 5; consequently, the principles of ray optics are used to describe trapping. In this region (known as Mie), the effects of diffraction are neglected and trapping forces are calculated considering the momentum impulse of light due to the reflection and refraction of rays at the surface of the sphere. Since the force on a dielectric object is given by the change in momentum of light induced due to refraction of the light by the object, the total force on the object is the difference between the momentum flux entering the object and that leaving the object. The object is pushed by the reflection of light from its surface while radiation forces due to refraction can be used to pull a transparent object.

In this study, a compact optical tweezers setup has been constructed on a 50x90 cm optical breadboard; the setup consists of a Sharp LT024 780 nm (near-infrared) diode laser, cylindrical and spherical lenses, various first-surface mirrors, and a commercial-quality 1000x oil-immersion microscope. Safety precautions have been taken into consideration in the design of the setup. There are many practical obstacles (mis-alignment of optical elements, laser beam distortions, etc) to achieving the theoretically predicted performance. Optimizing the trap efficiency is especially crucial when working with a relatively low power laser beam. Our setup uses a 20 mW laser (which is not much stronger than a laser pointer), while other research tweezers use lasers 10 - 100 times more powerful. For an optical trap to be stable, the beam must be symmetric about the direction of propagation; this is achieved by careful positioning of the telescoping lens systems and adjustment of the mirrors that lead to the microscope so the beam passes directly through the middle of each lens. Mounting the components as close as possible to the surface of the rigid optical breadboard minimizes changes in alignment due to vibration.

In a past study in this laboratory, modifying the laser beam profile has been shown to increase trapping efficiency, in spite of some loss of beam intensity. I hope to formulate a mathematical model for the dependence of trapping efficiency on beam intensity distribution, and to test this model with appropriate experiments; the light can be redistributed by simply blocking out portions of the beam, or by using an aspheric lens system or a diffractive element. Difficulties already encountered stem from the lack of a mature theoretical explanation of trapping in the complex-Mie region as well as the absence of a universal and effective method for quantifying trapping power.

This research was supported by the Simons Foundation and NSF Grant PHY 00-98044.

Challenges:

Procedures: Electronic Setup

Procedures: Turning on the sharp LT024 laser

Procedures: "Un-ellipticalizing" the Emergent Laser Beam

Procedures: Redirection of the the Laser Beam with Al coated first surface mirrors

Procedures: Resizing the Laser Beam

Procedures: Redirection of the Laser Beam (Periscope)

Procedures: Verticalization and Horizontalization of Beam

Figure 1: Correct Dichroic Mirror Angle

Figure 2: Incorrect Dichroic Mirror Tilt

Figure 3: Incorrect Dichroic Mirror Tilt

Figure 4: Dichroic Mirror Tilt Adjustment

Procedures: Rose Chamber Construction

Procedures: Measurement of Fine Adjustment Vertical Displacement :: Angular Degree Displacement Ratio

Figure 1: Microscope Setup

Figure 2: Degree Finder

Procedures: Viewing the Laser Beam

Procedures: Approximation of the Thickness of Melted Parafilm Wax

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